Sciencemadness Discussion Board

What's the minimum concentration for h2so4 to be oxidising?

fusso - 5-11-2019 at 18:35

What's the minimum concentration for h2so4 to be oxidising? (rxn to form SO2)
I can't find this info.

Amos - 5-11-2019 at 18:37

Depends on reducing agent, temperature, etc. Chemistry.

artemov - 6-11-2019 at 01:48

Sorry, just to tack on this thread ... what about the concentration to be dehydrating?
For example to dry liquid bromine or chlorine/SO2 gas? 80% ok?

Tsjerk - 6-11-2019 at 06:40

Quote: Originally posted by artemov  
Sorry, just to tack on this thread ... what about the concentration to be dehydrating?
For example to dry liquid bromine or chlorine/SO2 gas? 80% ok?


I guess somewhere around 37%, according to the graph attached, but how dehydrating do you want it?

[Edited on 6-11-2019 by Tsjerk]

Summary-of-endpoints-for-sulfuric-acid-droplet-cooling-experiments-plotted-on-H-2-SO-4-H.png - 31kB

woelen - 6-11-2019 at 07:36

To my experience, sulphuric acid starts to become somewhat oxidizing at a concentration of 60 to 65% by weight. If I mix equal volumes of water and 97% H2SO4 (the latter has 1.84 times the density of water) and I add this mix to e.g. KBr, then a little amount of Br2 is formed, while at 40% no Br2 is formed. Temperature has a strong influence. If the acid is warm, then more Br2 is formed.

Dehydrating power most likely requires higher concentration.

Amos - 6-11-2019 at 11:36

Quote: Originally posted by woelen  
To my experience, sulphuric acid starts to become somewhat oxidizing at a concentration of 60 to 65% by weight. If I mix equal volumes of water and 97% H2SO4 (the latter has 1.84 times the density of water) and I add this mix to e.g. KBr, then a little amount of Br2 is formed, while at 40% no Br2 is formed. Temperature has a strong influence. If the acid is warm, then more Br2 is formed.

Dehydrating power most likely requires higher concentration.


Have you done the same for iodide?

fusso - 6-11-2019 at 18:06

What about rxn with Cu?

AJKOER - 15-11-2019 at 19:02

Quote: Originally posted by fusso  
What about rxn with Cu?


Or with Hg, per comment:

"Concentrated sulphuric acid possesses marked oxidising power, especially in the presence of certain metallic salts, such as those of mercury and copper."

See comments at http://sulphur.atomistry.com/sulphuric_acid.html .

Sample reaction with concentrated acid:

Cu + 2H2SO4 → CuSO4 + SO2 + 2H2O

Source: https://chemiday.com/en/reaction/3-1-0-270

[Edited on 16-11-2019 by AJKOER]

fusso - 15-11-2019 at 19:44

Sigh. My question is the min conc for H2SO4 to be able to oxidise Cu.

vibbzlab - 15-11-2019 at 20:09

Do copper metal react in concentrated Sulfuric acid. I actually never tried that :P

Tellurium - 15-11-2019 at 20:42

Quote: Originally posted by vibbzlab  
Do copper metal react in concentrated Sulfuric acid. I actually never tried that :P

Yes it does react, at least when the sulfuric acid is heated. I saw a small blue coloration even without heating it, but I'm pretty shure, that this is just because of the oxide layer on the copper;)

woelen - 22-11-2019 at 07:26

For oxidation of copper metal you need highly concentrated acid (90+%) and even then, some gentle heating is necessary as well.

AJKOER - 4-12-2019 at 05:43

Quote: Originally posted by woelen  
For oxidation of copper metal you need highly concentrated acid (90+%) and even then, some gentle heating is necessary as well.


which I find very interesting given an alternate path that I have presented that does introduce, however, a sodium ion presence. A partial quote from https://www.sciencemadness.org/whisper/viewthread.php?tid=15... :
Quote: Originally posted by AJKOER  

.........................
Employing NaHSO4 as the source of the bisulfate, the net reaction would be:

Cu + H2O2 + 2 NaHSO4 ---Activated Carbon, Microwave---> CuSO4 + Na2SO4 + 2 H2O

where the expected k > 10^8 (based on p. 15 reported reactions by •SO4- acting on Fe(ll), Ce(lll), Cr(ll),.. at https://srd.nist.gov/NSRDS/NSRDS-NBS-65.pdf by Ross and Neta)

[Edited on 5-7-2019 by AJKOER]


However, the science I outlined in the referenced thread, is based on a process to radicalize the -HSO4 ion (and this can also be accomplished with even weak H2SO4 as a source of HSO4-). A relevant comment at https://www.sciencedirect.com/topics/earth-and-planetary-sci... :

"Sulfate radicals are one of the strongest oxidants available, with an oxidation potential of 2.6V, as compared to the hydroxyl radical (2.7V), the permanganate ion (1.4V), and ozone (2.2V) and are effective in oxidizing a broad range of chemical substances."

[Edited on 4-12-2019 by AJKOER]

Tsjerk - 4-12-2019 at 10:09

I know I shouldn't comment on AJKOER's comments, but AJKOER... Have you tried to perform the reaction you propose while taking the proper controls in account?

AJKOER - 4-12-2019 at 12:09

Quote: Originally posted by Tsjerk  
I know I shouldn't comment on AJKOER's comments, but AJKOER... Have you tried to perform the reaction you propose while taking the proper controls in account?


Well, we can discuss what constitutes good 'controls', but I simply reply, let's start by examining the reaction pictures provided in the link, and the amount of time involved to produce that degree of coloration.

I would also add that in the current context, the proposed reagents, in addition to pure copper metal, could include H2O2, dilute H2SO4 and a small amount of pure CuSO4 (as an electrolyte, or as pre-formed by adding, for example, CuO, from heated copper metal, or Cu(OH)2 or CuCO3 to H2SO4). The choice of the activated carbon source is a selection based on several factors (like accessibility, cost, purity requirements, review of the literature,....). High purity graphite rods (used in mechanical pencils) are likely also suitable here, perhaps crushed to increase surface area.

[Edited on 4-12-2019 by AJKOER]

AJKOER - 5-12-2019 at 09:59

Just found an interesting but old text accounting for the products of the action of H2SO4 on Cu, which can be a problematic reaction apparently due to a sulfide coating on the copper metal (alleviated by heating the reaction mix) and the evolution of SO2 at https://books.google.com/books?id=9SdZxFuyYGAC&pg=PA112&... .

For those seeking standard chemistry, apparently not so, with the proclaimed liberation of nascent hydrogen (more correctly the hydrogen atom or radical). This interestingly parallels the surface formation of .H cited by the action of Zn with HCl. However, in the latter case, there is no apparent action of .H on HCl as:

.H <--> H+ + e-

.H + H+ + Cl- = H+ + e- + H+ + Cl- = HCl + .H

[Edited on 5-12-2019 by AJKOER]

G-Coupled - 5-12-2019 at 21:14

Is the nascent Hydrogen made manifest via the medium of the aether?

clearly_not_atara - 5-12-2019 at 23:07

So my thinking here is, molecular H2SO4 is oxidizing, but HSO4- and H3O+ are not. We want to know the critical concentration of dihydrogen sulfate at which [H2SO4] is not very small. Since H2SO4 is strong in water, it’s probably close to (and definitely less than) a 1:1 molar ratio of H2SO4:water.

woelen - 5-12-2019 at 23:59

Indeed, H2SO4 is oxidizing, HSO4(-) is much less so and SO4(2-) is not. You need concentrations of 85+ % of H2SO4 in order to have reasonable amounts of H2SO4, otherwise you have mostly HSO4(-), which is less oxidizing. The latter, however, is not completely free of oxidizing properties. Mix a little KBr and NaHSO4 and heat in a test tube. You get mostly HBr, but also some Br2.

This behavior is quite common with oxo acids. A similar observation can be made with HNO3 and NO3(-), somewhat more pronounced than with H2SO4. Much more pronounced do we observe this with HClO4 and ClO4(-). Anhydrous HClO4 is insanely dangerous and reacts nearly explosively with many reductors (e.g. paper, sulphur, sugar, fabric, wood), while ClO4(-) is amazingly inert in aqueous solution (e.g. 70% acid does not react with iodide ion or with SO2 and when zinc metal or even magnesium metal is added to such acid you only get H2 and no reduction of ClO4(-)).

The reason behind the difference in oxidizing power between the molecular acid and the ionized acid is that in the ion, there is good symmetry and resonance stabilization. E.g. the outer electrons in ClO4(-) are equally distributed over all four oxygens. Similar stabilization also occurs in nitrate ion and sulfate ion. Salts of these acids, therefore also are stable. Sulfates, nitrates and perchlorates are stable salts for all metals and many, even strongly reducing, organic cations. On the other hand, esters of these acids, especially the nitrates and perchlorates, are very dangerous and can be made to explode by simply looking angry at them :P

AJKOER - 6-12-2019 at 18:12

Quote: Originally posted by G-Coupled  
Is the nascent Hydrogen made manifest via the medium of the aether?


Here is a more modern rendition, to quote a comment from a demonstration on 'Electrochemical Catalyst' supplied by Jeff Hughes at Applied Chemistry Department, RMIT at http://chem.lapeer.org/Chem1Docs/ElectChemCat.html which looks at the reaction between zinc metal and dilute sulfuric acid, without and with the presence copper metal (created in the cell), the latter promoting a favorable formation of an electrochemical cell.

Further, as to mechanics, the comment:

"The reason for the differences in overpotentials is related to the ease of formation and evolution of H2 bubbles, which occurs in several steps:- (i) diffusion of H+ to the metal surface; (ii) H+ + e => H; (iii) formation of a layer of M-H i.e. H atoms adsorbed on the metal surface; (iv) H + H => H2; (v) formation and evolution of a bubble from the surface"

Per above:
Quote: Originally posted by AJKOER  

.....................
I would also add that in the current context, the proposed reagents, in addition to pure copper metal, could include H2O2, dilute H2SO4 and a small amount of pure CuSO4 (as an electrolyte, or as pre-formed by adding, for example, CuO, from heated copper metal, or Cu(OH)2 or CuCO3 to H2SO4). The choice of the activated carbon source is a selection based on several factors (like accessibility, cost, purity requirements, review of the literature,....). High purity graphite rods (used in mechanical pencils) are likely also suitable here, perhaps crushed to increase surface area.


And:
Quote: Originally posted by AJKOER  

................
For those seeking standard chemistry, apparently not so, with the proclaimed liberation of nascent hydrogen (more correctly the hydrogen atom or radical). This interestingly parallels the surface formation of .H cited by the action of Zn with HCl. However, in the latter case, there is no apparent action of .H on HCl as:

.H <--> H+ + e-

.H + H+ + Cl- = H+ + e- + H+ + Cl- = HCl + .H


So, in my proposed reaction cell, copper and carbon are in an electrolyte likewise creating a beneficial electrochemical cell. Copper, the more reactive metal, is the anode, carbon is the cathode, so hydrogen ions are likely to be reduced to hydrogen atoms at the carbon electrode and may further react with H2O2 resulting in active radicals and associated products including CuSO4.

[Edited on 7-12-2019 by AJKOER]